The present invention relates generally to the field of electronic data storage. In particular, the present invention relates to a multiple-throated, single-coil, inductive servo writer for simultaneously writing multiple servo tracks on a magnetic tape medium.
Data are stored on magnetic tapes in parallel tracks that extend in the direction of the length of the magnetic tape. Historically, separate read and write heads existed for each distinct data track. Each of these read and write heads remained stationary while the tape scrolled past them. In this system, write-wide, read-narrow methodologies were sufficient to ensure that the read heads remained on-track during read-back.
To increase data densities on magnetic tapes, new head assemblies were developed that moved across the width of the magnetic tape, such that each read and write head in the head assembly would have access to multiple data tracks. To accurately position this type of head assembly, these tape drives relied upon servo schemes that had been used to control head positions in rigid disc applications. By using servo tracks, the head assembly could be placed in multiple positions or servo ranges across the width of the magnetic tape. The servo schemes used servo tracks consisting of a series of alternating magnetic moments on the tape. For instance, a conventional two servo scheme uses two servo tracks that are read by some number of servo readers to obtain multiple servo ranges across the width of a track. The number of available servo ranges in a conventional two servo scheme is 2(N-1), where N is the number of servo readers. Hence, a conventional two servo scheme employing four servo readers will provide six ranges of positions.
The center of each range is identified by two adjacent servo readers being positioned such that exactly one half of each servo reader is located directly above the same servo track. The border between ranges is located half way between centers. When the servo read signals from the corresponding servo readers are unequal, the servo loop moves the servo readers toward the center position. Each range of positions represents a set of data tracks, with each data track in a set associated with its own read head. Thus, if there are eight data read heads, a single servo range will be associated with eight data tracks.
In order to increase data density by increasing the number of tracks on a tape, it has been suggested that non-adjacent servo read heads, reading two separate servo tracks, could be used to define additional servo ranges. In the above stated example, one extra range could be gained if the read signal from separate servo tracks were used simultaneously to define the new range. The new range would be defined by the first servo read head covering one-half of the first track and the fourth servo read head covering one-half of the second servo track.
In such a system, however, any discrepancy in the two servo tracks results in tracking problems. In particular, phase shifts between the alternating patterns in the two servo tracks make it impossible to determine the head assemblies' position within the added range. In fact, a phase shift between the two servo tracks can cause ringing, where the head assembly oscillates across the range without finding a fixed range center. This oscillation occurs because the phase shift introduces an oscillating inequality between the read signals of the two servo heads.
Currently, there are two methods in the prior art for producing parallel servo tracks. In one method, the servo tracks are written by one servo writer which writes one servo track before being moved a small distance across the width of the magnetic tape to write additional servo tracks. In this method, small changes in the tape speed, or flutter, will cause the transitions in the servo tracks to be out-of-phase with each other. In the second method of creating parallel servo tracks, independent servo writers are used in parallel. However, in this method minute differences between the independent heads cause the transitions in the servo tracks to be out of phase. Even heads fabricated on the same wafer will have variations in thickness of the individual layers that can cause a phase shift between the servo tracks.